DOI: 10.1590/2317-4889201820170090 ARTICLE Low-temperature metamorphism in the Capiru Formation, Morro Grande Synform, Southern Ribeira Belt Larissa da Rocha Santos1*, Renato Leandro2, Anelize Bahniuk3, Leonardo Fadel Cury4 ABSTRACT: The Capiru Formation consists of a low-grade meta-sedimentary succession of slates, phyllites, quartzites and marbles, disposed in blocks delimited by thrust and strike-slip faults in the Curitiba Terrane, Southern Ribeira Belt, Southern Brazil. The metamorphic and deformation records are heterogeneous, with deformed zones tectonically interbedded by domains with preserved sedimentary features. The present work aims to understand the development of mineral paragenesis based on geochemical and petrographic analysis. We selected samples of metasandstones, slates, phyllites and rhythmic phyllites, with different metamorphic and deformation records to be analysed. We selected samples with different metamorphic records to be analysed. The petrographic characterization of foliations was made through field studies supported by microtectonic analysis. The mineralogical and chemical compositions were determined by X-ray diffractometry and X-ray fluorescence, respectively. The results show a preserved sedimentary bedding0 (S ), defined by sedimentary structures with top-and-bottom indicators. The S1 foliation is related to a thrust tectonics, and it is characterized by a continuous slaty cleavage. The 1S slaty cleavage is crenulated and cut by a millimetric S2 cleavage. The mineral assemblage is composed by quartz, sericite, magnetite/ goethite and carbonaceous material. The metamorphism was developed under low-temperature conditions (between 250–350ºC) and low pressure gradients (2.5–4.5 kbar). The geotectonic environment is considered as a thrust-and-fold-belt system, with structures developed in superior crust levels. KEYWORDS: Capiru Formation; low-grade metamorphism; Southern Ribeira Belt; tectonic evolution. INTRODUCTION fine-grained scale of the products. Also, the apparent lack of regularity in the metamorphic development and the partial Integrated studies of metamorphic petrology and struc- recrystallization with many relict features of the protolith tural geology are important tools to understand the tectonic point to non-equilibrated systems without obvious overall evolution in orogenic interior zones. However, relationships controls (Robinson & Merriman 1999). Given these facts, between metamorphic mineral growth, different deforma- the key to study low-grade terrains take into consideration tion phases and the control of developed tectonic structures metamorphic reaction progress and the original minerals is challenging in low-grade terrains (Poyatos et al. 2001). transformation within their respective polytypes. Rocks formed in low-grade metamorphic conditions are Metamorphic petrology techniques by optical micros- commonly identified by the abundance of hydrated miner- copy combined with geochemical analysis that quantifies the als, fine grains, disordered crystallized structures, with con- progression of metamorphic reactions could provide infor- siderable compositional variation that represents chemically mation on orogenic complexes and their different portions and structurally metastable phases in the phyllosilicates (Sassi in a structural context. Correlations between deformation, & Scolari 1974). reaction progress and metamorphic grade in deformed areas In terms of low-temperature metamorphism, the recrys- under low- and very low-grade metamorphism have been tallization is often not immediately obvious due to the examined by some authors. Roberts and Merriman (1985) 1Programa de Pós-graduação em Geologia Exploratória, Departamento de Geologia, Laboratório de Minerais e Rochas – Universidade Federal do Paraná – Curitiba (PR), Brasil. E-mail: [email protected] 2Programa de Pós-graduação em Geologia Exploratória, Departamento de Geologia, Universidade Federal do Paraná – Curitiba (PR), Brasil. E-mail: [email protected] 3Departamento de Geologia, Laboratório de Minerais e Rochas – Universidade Federal do Paraná – Curitiba (PR), Brasil. E-mail: [email protected] 4Departamento de Geologia, Laboratório de Minerais e Rochas – Universidade Federal do Paraná – Curitiba (PR), Brasil. E-mail: [email protected] *Corresponding author. Manuscript ID: 20170090. Received on: 07/08/2017. Approved on: 01/07/2018. 95 Brazilian Journal of Geology, 48(1): 95-113, March 2018 Low-temperature metamorphism in the Capiru Fm interpreted a causative relationship between strain and low GEOLOGICAL SETTING metamorphic grade from the association with an anticline geometry. Gutiérrez-Alonso and Nieto (1996) established The Capiru Formation, defined by Bigarella and Salamuni a semi-quantitative relationship between illite cristallinity (1956), is a supracrustal unit of the Curitiba Terrain (Siga Jr. and finite strain, related to the distance from an important 1995). It is characterized by lithologic units composed of meta thrust across a large Variscan structure. psammites, metapelites, and metadolomites product of low- Poyatos et al. (2001) obtained X-ray diffraction data sug- grade metamorphism, disposed in blocks delimited by signif- gesting that the rocks did not reach the equilibrium, but strain icant thrust faults (Fiori 1991, 1994, Fiori & Gaspar 1993). effects favored the progress of mineral reaction in deformed The rocks of the Capiru Formation crop out as folded sectors, from syn-kinematic studies of the very low-grade and elongated zones, mostly in NE-SW trending, bounded metamorphism in a polyphase deformed Variscan sector. on the north by the Lancinha Shear Zone (LSZ), and to the Potel et al. (2006) investigate the very low-grade south by the Atuba Complex rocks (Fig. 1). metapelites of the Koumac and Diahot terrains in New There are at least two tectonic-metamorphic events in Caledonia, using the illite cristallinity and bo cell dimen- the Capiru Formation (Fiori 1991). One was related to a sions of K-white micas, and demonstrate that the bo cell thrust tectonics and another related to a transpressive tec- dimension provides a robust estimate of maximum pres- tonics with strike-slip shear zones and their related struc- sure reached in low-temperature domains with polyphasic tures (Fiori 1991). metamorphic histories. The thrust tectonic event was responsible for the devel- The aim of this study is to estimate the local conditions opment of a regional phase of progressive metamorphism of low-grade metamorphism in a polyphasic deformed area under greenschist facies conditions (biotite zone), whereas in the Capiru Formation, at Morro Grande region, empha- the transpressive event deformed the previous records in a sizing the analytical techniques applied with the purpose of large scale fold system, which generate paregenesis restricted understanding the early stages of metamorphic development, to the mylonite zones (Leandro 2016). related to the tectonic and structural contexts in which these The metasedimentary rocks of the Capiru Formation rocks were formed. display intensely deformed tectonofacies contrasting with Quantitative studies of progressive metamorphic reac- preserved stratigraphic successions, containing relict sed- tions in low-grade rocks should take into account a number imentary structures, such as those described by Bigarella of variables influencing the results: compositional variations and Salamuni (1956), Fiori and Gaspar (1993), Juschaks of protoliths, chemical availability, amount of circulating (2006), Bahniuk (2007), Silva (2010), Leandro (2016), and fluids, porosity and permeability; their association with Leandro et al. (2017). tectonic structures should be well established, in order to Based on geochronological studies, Leandro (2016) con- understand the main mechanisms that affect the reaction siders the Capiru Formation as an allochthonous unit in a progress (Frey 1991). Due to the complexities of under- thrust-and-fold belt context. U-Pb ages obtained in detrital standing the combination of main controls, studies for this zircons from the Morro Grande unit show strong peaks at purpose should be extremely detailed. 2.1 Ga. However, the weighted mean age of the youngest con- For this study, two detailed cross-sections were chosen cordant zircons yield maximum deposition ages of 1.08 Ga. aiming to understand the early stages of metamorphic devel- opment. Differences in deformation intensity or deformation Morro Grande Synform (MGS) structure phases developed in a progressive system can be recognized in The MGS rocks crop out on the north side of the city of the cross-sections, providing a wide range of contingencies to test Colombo, southern Brazil (Fig. 1). The synform presents a correlations between crystal-chemical data, organic matter mat- W-E trend, with 70 km2 of exposure. The axial plane shows uration index and petrographic features under local conditions. approximate N70 strike and moderate dip angles around The selected sites for the detailed study were the preserved 60º towards SE, comprising a regional synform with an stratigraphic succession sectors within a well-known structure inverted flank (Fiori 1991). in Capiru Formation, the Morro Grande Synform (MGS). The Capiru Formation is subdivided, bottom-to-top, The MGS, extensively studied by Fiori (1991, 1994), Fiori and in three lithological sets: Juruqui, Rio Branco and Morro Gaspar (1993) and Leandro (2016), consists of two large thrust Grande, delimited by thrust faults. The stratigraphic sequence units,